Rats with lesions in the medial preoptic area were placed in a thermocline where they could choose ambient temperatures (Ta) ranging from 17 to 31 degrees C. The hypothalamic temperature (Thy) and the selected Ta were continuously recorded. Amplitude of Thy circadian oscillations was exaggerated in 23 out of 27 lesioned animals. Only in 6 circadian cycles out of the 158 recorded, the rats chose a higher Ta during the night. In 55% of the recordings the preferred Ta was significantly lower during the night than during the day and out of phase with Thy. In the other 36% of the recorded cycles no significant difference was found between the night and day preferred Ta. Therefore in 91% of the recorded cycles the preferred Ta oscillations did not support the idea that the exaggerated circadian oscillations of Thy in lesioned rats would be due to exaggerated cyclical shifts of the set-point temperature. The lesions induced many sporadic anomalies in the behavior of the rats in choosing a preferred Ta. Short episodes of almost parallel variations in Thy and Ta in phase with each other suggest that some lesions induce an instability of the set-point mechanism.